This invention relates to mechanisms for changing gear speed ratios between selective values. More particularly, this invention relates to speed change mechanisms including gear change yokes actuated by travelling ball nuts on screw shafts.
Prior art devices of the type referenced herein are subject to frequent failures resulting from premature wear of the screw shaft. The latter is induced by freewheeling of the shaft at the end-stroke limits, whereupon the balls rub against the screw shafts as the screw shaft winds to a stop. This occurs because the screw shaft turns without effecting axial movement of the travelling ball nut. Resultant ball and screw wear progresses, the balls decreasing in size and contact stresses becoming even greater. Ultimate deterioration finally results in failure of the screw assembly to traverse, thus causing motor burn-out.